The present invention relates to a process for producing a sheet of a thermoplastic which is biaxially stretched at temperatures above the glass transition temperature and heatset, to a sheet produced by the process, and to a laminated safety glass which includes the sheet.
Sheets of thermoplastic materials are frequently joined to other layer materials to form composites. For example, sheets of polyethylene terephthalate are used as supports for function layers. These coated sheets are sandwiched between two sheets of polyvinylbutyral and integrated into laminated safety glass panes.
Laminated safety glass is one of the glazing materials frequently used today. For example, a large proportion of motor vehicle windshields is composed of laminated safety glass. Laminated safety glass is also used in glazing buildings.
In general, conventional laminated safety glass consists of three layers, two glass panes being joined to each other by a sheet of polyvinylbutyral. Such glass laminates offer a high measure of safety since, in the event of an impact loading, the sheet of polyvinylbutyral elastically absorbs mechanical energy, and glass splinters which may be produced remain stuck to the sheet.
The requirements which are imposed presently on laminated safety glass are not, however, limited to the safety aspect.
In many cases it is desirable for laminated safety glasses of this type to fulfill additional functions. It may, for example, be necessary for the pane of laminated safety glass to offer a special heat protection or protection against strong solar radiation. Further requirements, which are imposed, for example, specifically on motor vehicle windshields, are the capability of being electrically heated and the possibility of incorporating so-called head-up displays.
One possibility for equipping laminated safety glass with additional functions is to integrate thin layers into the glass laminate. This can be done by depositing one or more function layers on that side of one of the two glass panes which faces the polyvinylbutyral sheet in the subsequent lamination process. To coat individual glass panes is very complicated and expensive. In producing windshields for motor vehicles, there is the additional difficulty that the coated, initially flat panes also have to pass through a bending process while being heated, and this easily leads to tearing of the function layers. In this critical operational step, the yield of acceptable windshields is low.
It is cheaper to integrate thin function layers into the glass laminates by first depositing the layers in a continuous process on a hightransparency substrate sheet and then incorporating the coated substrate sheet into the glass laminate.
A suitable material for the substrate sheet is polyethylene terephthalate. The coated substrate sheet is embedded between two sheets of polyvinylbutyral and laminated safety glass panes are thus produced which have the structure: glass/PVB sheet/(coated sheet)/PVB sheet/glass. If commercially available polyethylene terephthalate sheets are used, the coated substrate sheet is observed to form corrugations in the finished glass laminates. This has a highly disturbing effect on the appearance of the glass laminates and, what is more, objects viewed through a glass laminate of this kind appear distorted. Use of these glass laminates is out of the question.
European Patent No. 0,077,672 proposes a solution to this problem. The essential idea in this patent is that only those polyethylene terephthalate sheets which have very specific thermomechanical properties are used as the substrate sheet. This means that the thermal shrinkage E of a sheet and its thickness d fulfil the following relationship: EQU 4.4.gtoreq.E.gtoreq.0.00028.times.(d-128).sup.2 ( 1) EQU d&lt;125 (2)
Here E stands for the thermal shrinkage measured in % which sets in after a 30-minute temperature stressing of 120.degree. C., and d for the sheet thickness measured in .mu.m. The reason for the restriction of the permitted thermal shrinkage to the interval specified in (1) is that, on the one hand, a higher shrinkage results in damage to the function layer and on the other hand, the small flatness faults which always occur in the sheet can no longer be smoothed out with a lower shrinkage.
It was found, however, that it is difficult to produce glass laminates which have a satisfactory appearance even with substrate sheets which fulfil the two relationships (1) and (2). In particular, sheets which fall into the boundary range of the region of the E-d plane defined by the relationships (1) and (2) result in a damaged function layer or in flatness faults in the glass laminates.
The relationship (1) implies that the thinner the sheet is, the more accurately the shrinkage of the sheet has to be adjusted. For sheets having a thickness of d=12 .mu.m, (1) and (2) yield, for example, a shrinkage in the range 4.4&gt;E&gt;3.7. Sheets having a thickness of d=2.6 .mu.m and less are not encompassed by relationship (1). In the patent mentioned, the range EQU 3.9.gtoreq.E.gtoreq.0.00028.times.(d-130).sup.2 ( 3) EQU d&lt;125
is described as particularly favorable. Even sheets having a thickness of d less than or equal to 12 .mu.m are not encompassed by relationship (3).
In most processes for producing composite bodies from sheet-shaped structures, the individual layer materials are first superimposed or united and then subjected to temperature and pressure loading. During heating, films of thermoplastic materials first expand and this causes a corrugation formation in the composite body. To smooth the corrugations again, shrinkage starting at higher temperatures must be very high, but this will then result in damage to the function layer. Dimensional changes which do not damage the function layer are generally insufficient to smooth the corrugations produced by thermal expansion.
German patent application P 39 04 191.3 describes a process for producing a glass laminate comprised of glass, polyvinylbutyral sheet, and coated plastic sheet. Polyethylene terephthalate sheets having specific shrinkage properties and also any commercial high-transparency plastic sheets possessing thermal shrinkage characteristics can be used as the support sheet for the function layer. During the laminating procedure of this process care is taken that the area-related dimension of the sheet carrying the function layer is not reduced. This is achieved by clamping the sheet in a clamping device during the laminating procedure.
This method of producing a laminated safety glass is expensive. Particularly in spherically curved glass panes with complicated edge lines it is difficult to secure the sheet evenly on all four sides during the laminating procedure.